The present invention relates to an apparatus for photocatalytic reaction for efficiently purifying organic compounds or nitrogen oxides contained in water or wastewater (solution to be treated) by utilizing a photocatalyst and a photocatalyst immobilizing method for efficiently immobilizing the photocatalyst used in the apparatus for photocatalytic reaction on a surface of a support.
In recent years, attention has been focused on environmental problems and emphasis has been laid on tightening of regulations of wastewater, exhaust gases and the like. As part of it, a technique for purifying the wastewater or the exhaust gases has been remarkably highlighted.
By way of examples, a variety of attempts have been made to make the wastewater or exhaust gases containing these hazardous compounds nonhazardous by so-called xe2x80x9cphotoreactionxe2x80x9d such as photodegradation or photooxidation caused by irradiating the organic compounds or nitrogen oxides (N Ox) with light having a specific wavelength.
It is known that the photoreaction (photoreaction using the photocatalyst is referred to as xe2x80x9cphotocatalytic reactionxe2x80x9d) can efficiently occur when titanium dioxide, zirconium oxide, or the like is used as a catalyst (photocatalyst) to cause the photoreaction to occur. It is also known that familiar solar light, for example, light of a fluorescent lamp or the like can be used as light.
Specifically, a purifying method for making the hazardous compounds nonhazardous has been proposed, in which a powdered photocatalyst is put into the solution to be treated containing the hazardous compounds such as the organic compounds or the nitrogen oxides (N Ox)and suspended, and the solution to be treated, to be precise, the photocatalyst in the solution to be treated is irradiated with light.
However, this purifying method has a technical drawback, because the powdered photocatalyst is suspended in the solution to be treated after an elapse of long time, it is difficult to efficiently collect the photocatalyst after treatment.
Accordingly, to eliminate the necessity of collecting the powdered photocatalyst after photoreaction, a purifying method has been proposed, in which the powdered photocatalyst is immobilized on a support (glass plate and the like) by using a binding agent comprising organic high molecular compounds or inorganic compounds and irradiated with light from a light source placed in atmosphere with the support immersed in the solution to be treated.
However, in this purifying method, since it is difficult for the light to reach a photocatalyst surface on the support surface immersed in the water, the photocatalytic reaction is degraded and treatment efficiency is reduced. On the other hand, a method for immersing the light source as well as the photocatalyst has been proposed, to improve the photocatalytic reaction by making the light source closer to the photocatalyst. In this case, it is necessary to cover a surface of the light source with a glass pipe or the like. Hence, this method also has a technical drawback, because with an elapse of immersion time, foreign substances are gradually attached to the surface of the glass pipe, which causes the treatment ability to be degraded with an elapse of time.
By the way, when the powdered photocatalyst is immobilized on the support (glass plate or the like) by using the binding agent comprising the organic high molecular compounds, the binding agent is expensive and tends to be deteriorated by photodegradation or photooxidation. Also, the photocatalyst surface is covered with the binding agent, which causes a catalytic function to be degraded.
Accordingly, as described in Japanese laid-Open Patent Publication No. Hei. 7-126324, a method has been proposed, in which an aqueous solution in which alkali metal silicate of an inorganic compound which is less subject to photodegradation or photooxidation and the powdered photocatalyst are mixed is coated on a surface of a support such as glass (including an inner face of a container into which wastewater is put) and used as the photocatalyst.
In this case, however, the photocatalyst is covered with the alkali metal silicate acting as the binding agent, or coating becomes difficult depending on the ratio of the alkali metal silicate to the photocatalyst, which is a serious drawback.
In consideration of the above circumstances, an object of a first invention is to provide an apparatus for photocatalytic reaction that has high treatment efficiency and treatment ability that is not degraded with an elapse of operating time and an object of a second invention is to provide a method for simply and efficiently immobilizing the photocatalyst on the support.
To solve the above-described technical problems, an apparatus for photocatalytic reaction according to the first invention is characterized in that a drum having a photocatalyst immobilized on a surface thereof is rotated with a part of the drum immersed in a solution to be treated and a part of the drum exposed on the solution to be treated so that immersion and exposure are continuously repeated and an exposed surface of the drum is irradiated with light for causing the photocatalytic reaction to occur.
According to the apparatus for photocatalytic reaction configured described above, because the part of the drum is immersed in the solution, the solution to be treated is attached in thin film shape to the photocatalyst on the surface of the drum and irradiated with light in atmosphere outside of the solution. Therefore, the photocatalytic reaction can always occur efficiently and continuously. Also, the photocatalytic reaction can be realized with a relatively simple configuration.
An embodiment of the apparatus for photocatalytic reaction according to the first invention is that a conical plate having an increased thickness at center of rotation and a reduced thickness on an outer peripheral side thereof and having a photocatalyst-immobilized surface is rotated with a part of the conical plate immersed in the solution to be treated and a part of the conical plate exposed on the solution to be treated so that immersion and exposure are continuously repeated and an exposed surface of the conical plate is irradiated with light for causing the photocatalytic reaction to occur. With this configuration, the photocatalytic reaction can occur if the light for causing the photocatalytic reaction to occur is emitted from above.
Further, another embodiment is that a plate having a photocatalyst immobilized on a surface thereof is continuously and repeatedly immersed in the solution to be treated and exposed on the solution to be treated and an exposed plate surface is irradiated with light for causing the photocatalytic reaction to occur. In this case, a plurality of plates are placed around a rotating shaft like a water wheel to make surface areas larger.
A further embodiment is that the photocatalyst immobilized on the surface is hydrophilic. This embodiment is preferable in that the solution to be treated is reliably attached in thin film shape on the surface.
A still further embodiment is that platinum functioning as a catalyst is attached on the photocatalyst surface immobilized on the surface. Thereby, if oxygen dissolved in the solution to be treated is less, the photocatalytic reaction occurs actively and stably. When the solution to be treated is provided in thin film shape, oxygen (air) in atmosphere can be utilized and attachment of platinum is not always necessary.
To solve the above-described technical problem, a method for immobilizing a photocatalyst according to the second invention in which the photocatalyst is immobilized on a support by using alkali metal silicate as a binding agent, comprises: a first step for coating an aqueous solution of the alkali metal silicate on the support and immobilizing the aqueous solution thereon, a second step for coating a powdered photocatalyst on the alkali metal silicate and immobilizing the powdered photocatalyst thereon after the first step, and a third step for curing the coated and immobilized photocatalyst after the second step.
According to the method for immobilizing the photocatalyst having such a procedure, the powdered photocatalyst with high performance is efficiently and stably immobilized on the surface of the support. Therefore, so fabricated photocatalyst on the surface of the support is capable of making hazardous compounds nonhazardous by photoreaction for a long period of time without being dropped off or peeled off after use.
According to the method of the second invention, the photocatalyst can be coated on the surface of the support with highest efficiency in fabrication. Therefore, this can be mass-produced in a short time and at a low cost.
In an embodiment of the method for immobilizing photocatalyst according to the second invention, in practice, the aqueous solution of the alkali metal silicate used in the first step is preferably an aqueous solution having a concentration of 1 wt %-60 wt % and the photocatalyst used for coating in the second step is preferably a powdered photocatalyst or an aqueous solution having a concentration of 2 wt % or more.
In this case, the addition of the 0.02 wt %-1 wt % alkali metal silicate to the aqueous solution reduces viscosity and makes it difficult for a nozzle to be clogged when the aqueous solution is applied by spraying, which is a preferable embodiment.
Another embodiment is that immobilization in the first and second steps may be dry processes and curing in the third step may be a sintering process.
In this case, a preferred embodiment is that the support is made of a heat-resistant material, for example, glass.
Preferably, a further embodiment is that the immobilization in the first step is a dry process at a temperature within a range of an ordinary temperature to 150xc2x0 C., the immobilization in the second step is a dry process at a temperature within a range of the ordinary temperature to 150xc2x0 C., and the curing in the third step is a sintering process at a temperature of 200xc2x0 C.-800xc2x0 C.
This embodiment is preferable in that the photocatalyst can be stabilized on the surface of the support for a long period of time.
While a thickness of a photocatalytic layer formed on the support is not limited in the second invention, it is preferable that the thickness of the photocatalytic layer is approximately 0.1 xcexcm-500 xcexcm in terms of sufficient absorption of irradiation light.
The support may have an arbitrary configuration, including a plate, a sphere, granule, a cylinder, fibers, a woven fabric, and a non-woven fabric. The support may be generally made of a material such as plastic, glass, metal, ceramic.
When the hazardous compounds of the organic compounds or the nitrogen oxides (Nox) are degraded or photooxided by using the photocatalyst immobilized on the support according to the method described above, the solution or gas containing the hazardous compounds is contact with the photocatalytic layer under irradiation of near ultraviolet light. Preferably, the wavelength of the light is varied according to the type of the photocatalyst in terms of efficiency. For example, when the photocatalyst is titanium dioxide, near ultraviolet light including light of a wavelength of 90 nm or smaller is preferable. Therefore, preferably, a high pressure or low pressure mercury lamp, a xenon lamp, a black light, or the like is used as the light source. When the catalyst is tungsten oxide, a tungsten lamp as well as the above light sources is a preferable light source. Solar light is effective as the irradiation light. Also, a familiar light source, i.e., a fluorescent lamp may be used.
The immobilized photocatalyst according to the second invention is capable of degrading various types of hazardous compounds contained in the solution or gas and making them nonhazardous like the conventional powdered photocatalyst. For example, the photocatalyst is capable of degrading aromatic compounds such as picric acid or phenol, organic chlorine compounds, agricultural chemicals, surfactant in wastewater, and benzene, toluene, Nox in polluted air and making them nonhazardous.